This invention relates generally to communications networks, and more particularly to decreasing power consumption of nodes of the network that have a limited capacity power source.
Communications devices, such as pagers, cellular telephones, and personal digital assistants (PDAs), generally xe2x80x9cnodes,xe2x80x9d can increase their useful life by reducing power consumption when the devices are not in use. Power reduction is particularly important now that portable devices are coming into common use. Limited capacity power sources can include batteries, solar cells, thermocouples, piezo crystals, miniature turbines, and the like.
For example, a node is in a xe2x80x9csleepxe2x80x9d mode with minimal power consumption until the node actually needs to receive or transmit data in an xe2x80x9cawakexe2x80x9d mode. One prior art power saving protocol is standardized in the IEEE 802.11 specification.
It would be desirable that the nodes in the network agree on a sleep period and an awake period, and defer all power consuming activities until the awake period. As an example of how this might work, each node is in sleep mode the first 98% of each interval, and checks for communications during the last 2% of each interval. If data need to be communicated, then the devices can extend their awake period until all data are transmitted and received. Of course, this agreement requires the synchronization in time of all nodes in the network.
Prior art approaches have usually generated the required time synchronization signal in the network itself. Often, one node of the network, a xe2x80x9csynchronizationxe2x80x9d node sometimes called an access point, broadcasts a timing signal at regular intervals. The timing signal can be used by all the other nodes to synchronize to the access point, and hence to each other. This scheme is quite feasible when the access point is a high-power transmitter not necessarily concerned about reducing power consumption.
Unfortunately, the broadcast of a regular time signal presents problems for low-power and ad-hoc networks. Ad-hoc refers to networks with mobile nodes that do not rely on a stationary or ever-present infrastructure, for example, a network where all nodes are powered by limited capacity power sources, and the topology of the network is indeterminate and without central management. For example, personal digital assistants (PDA) having functionality equivalent or better than personal computers of the not so distant past are coming into common use. If the PDAs could be configured as loosely coupled networks for data interchange, then their utility would be increased.
As a first problem, the broadcasting of a timing signal at regular intervals must continue even when the network has no data to interchange, needlessly drawing power in the node responsible for generating the timing signal. Second, the network must agree on which node will broadcast the time signal. In ad-hoc networks, with no infrastructure, where mobile nodes can enter and leave at will, maintaining a timing signal by negotiation is a high-power activity that has to happen whenever a node moves, even when there are no data to interchange.
Therefore, it is desired that the synchronization of nodes in a network can be accomplished without having a particular node of the network be designated as synchronization node. In addition, synchronization of all network nodes should be possible without centralized management. If nodes can be synchronized without using network resources, then the consumption of power can be reduced for all nodes.
The invention provides means and a method for reducing power consumption in a communications network including a plurality of limited power capacity devices such as personal digital assistants. Each device periodically receives a timing signal from a transmitter external to the network. A real-time clock in each device is synchronized to the periodically received timing signal. The real-time clock determining a basic synchronized timing interval.
The transmitting and receiving of data between the devices is initiated during an awake period of the synchronized timing interval, and power consumption is reduced during a sleep period. The awake and sleep periods are synchronized to the basic timing interval. In order to minimize power consumption, the sleep period is significantly longer than the awake period.